Remove pending_consumers_ from producer channel

pending_consumers_ was created before the creation of buffer state atomics.
Producer channel need to check and issue signal if pending_consumers_ is
zero. pending_consumers_ == 0 is the same as buffer_state of
non-orphaned buffers are fully released. Thus, removing
pending_consumers_ as it gives redundant information and lower the
complexity of producer channel.

Bug: 119331650
Test: all tests are passing
on Blueline:
AHardwareBufferTest BufferHubBuffer_test BufferHubMetadata_test
buffer_hub_binder_service-test buffer_hub_queue_producer-test
libgui_test libsensor_test vrflinger_test buffer_hub-test
buffer_hub_queue-test dvr_buffer_queue-test dvr_api-test
dvr_display-test

on Vega:
AHardwareBufferTest BufferHubBuffer_test BufferHubMetadata_test
buffer_hub_queue_producer-test  buffer_hub-test buffer_hub_queue-test
dvr_buffer_queue-test dvr_api-test libdvrtracking-test

Change-Id: I5ccf3e6b3a2a304e238c272bda443e6d180babc3